Easy question

Your typical aquarium will be made with height as its main attribute for the sake of being a "display tank".

A breeder tank is made with the intention of practicality. The vast majority of fish swim horizontally without realizing they have lateral space to use.

A breeder tank is far more healthy for the fish's mental state, but to the common eye a display tank looks nicer. But this isn't necessarily a clear cut design. For instance, I think a 125g (6ftx2ftx1.5ft) is one of the most well-proportioned tanks you can get but still works well as a display.
 
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